A Glimpse of Satellite Galaxies in the Milky Way with the 2.5-meter Wide Field Survey Telescope (WFST): Bootes III and Draco

TL;DR

This study demonstrates WFST's capability to measure proper motions of faint stars in Milky Way satellite galaxies, providing insights into their dynamics and origins, exemplified by Bootes III and Draco.

Contribution

First application of WFST for proper motion measurements of faint stars in satellite galaxies, revealing their kinematic properties and potential origins.

Findings

Proper motions of stars in Bootes III and Draco measured with ~1.8-3.0 mas/yr accuracy.

Bootes III likely a bound remnant of a progenitor galaxy linked to the Styx stream.

No significant proper motion variation across stellar luminosities in these galaxies.

Abstract

We carry out deep imaging of the Milky Way satellite galaxies, Bootes III and Draco, with WFST as one pilot observing program to demonstrate the capability of WFST. Combining catalogs with PS1 DR2 and Gaia DR3, we derive proper motions for candidate member stars in these two satellite galaxies over a 12-year time baseline, yielding uncertainties of ~1.8 mas/yr at 21 mag and ~3.0 mas/yr at 22 mag in the r band. The proper motions derived from bright and faint stars are consistent, indicating no significant variation in proper motion across stellar luminosity as these galaxies undergo tidal interactions with the MW. Meanwhile, we suggest that Bootes III represents the bound remnant of the progenitor galaxy that gave rise to the Styx stream, as evidenced by its elongated density profile and overdensity in both spatial and kinematic space. This is the first paper to use WFST to measure the proper motions of faint stars in Milky Way satellite galaxies. More detailed analyses will be presented in forthcoming papers from the wide field survey (WFS) program.